Multidentate (18)F-polypegylated styrylpyridines as imaging agents for Aβ plaques in cerebral amyloid angiopathy (CAA)

Design, synthesis, and biological evaluation of multiple F-18 S1PR1 radiotracers in rodent and nonhuman primate

Here we report our design and synthesis of 28 new fluorine-containing compounds as potential F-18 radiotracers for CNS imaging of sphingosine-1-phosphate receptor 1 (S1PR1), and determination of their in vitro binding potency and selectivity toward S1PR1 over other S1PR subtypes. Nine potent and selective compounds, 7c&d, 9a&c, 12b, 15b, and 18a-c with IC50 values ranging from 0.6-12.3 nM for S1PR1 and weak binding toward S1PR2, 3, 4, and 5, were further 18F-radiolabeled to produce [18F]7c&d, [18F]9a&c, [18F]12b, [18F]15b, and [18F]18a-c. Multi-step F-18 radiochemistry procedures were investigated for radiosynthesis of [18F]7c&d and [18F]9a&c, and the presumed intermediates were synthesized and authenticated by analytic HPLC. We then performed nonhuman primate (NHP) PET brain imaging studies for eight radiotracers: [18F]7c&d, [18F]9a, [18F]12b, [18F]15b, and [18F]18a-c. Three radiotracers, [18F]7c, [18F]7d, and [18F]15b, had high NHP brain uptake with standardized uptake values (SUVs) at 2 h post-injection of 2.42, 2.84, and 2.00, respectively, and good brain retention. Our ex vivo biodistribution study in rats confirmed [18F]7d had a high brain uptake with no in vivo defluorination. Radiometabolic analysis of [18F]7c and [18F]7d in rat plasma and brain samples found that [18F]7c has a more favorable metabolic profile than [18F]7d. However, the trend of increased brain uptake precludes [18F]7c as a suitable PET radiotracer for imaging S1PR1 in the brain. Further structural optmization is warranted to identify a highly S1PR1-specific radiotracer with rapid brain uptake kinetics.

A closer look at amyloid ligands, and what they tell us about protein aggregates

The accumulation of amyloid fibrils is characteristic of neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease. Detecting these fibrils with fluorescent or radiolabelled ligands is one strategy for diagnosing and better understanding these diseases. A vast number of amyloid-binding ligands have been reported in the literature as a result. To obtain a better understanding of how amyloid ligands bind, we have compiled a database of 3457 experimental dissociation constants for 2076 unique amyloid-binding ligands. These ligands target Aβ, tau, or αSyn fibrils, as well as relevant biological samples including AD brain homogenates. From this database significant variation in the reported dissociation constants of ligands was found, possibly due to differences in the morphology of the fibrils being studied. Ligands were also found to bind to Aβ(1-40) and Aβ(1-42) fibrils with similar affinities, whereas a greater difference was found for binding to Aβ and tau or αSyn fibrils. Next, the binding of ligands to fibrils was shown to be largely limited by the hydrophobic effect. Some Aβ ligands do not fit into this hydrophobicity-limited model, suggesting that polar interactions can play an important role when binding to this target. Finally several binding site models were outlined for amyloid fibrils that describe what ligands target what binding sites. These models provide a foundation for interpreting and designing site-specific binding assays.

Tracer development for PET imaging of proteinopathies

This review outlines small molecule radiotracers developed for positron emission tomography (PET) imaging of proteinopathies, neurodegenerative diseases characterised by accumulation of malformed proteins, over the last two decades with the focus on radioligands that have progressed to clinical studies. Introduction provides a short summary of proteinopathy targets used for PET imaging, including vastly studied proteins Aβ and tau and emerging α-synuclein. In the main section, clinically relevant Aβ and tau radioligand classes and their properties are discussed, including an overview of lead compounds and radioligand candidates studied as α-synuclein imaging agents in the early discovery and preclinical development phase. Lastly, the specific challenges and future directions in proteinopathy radioligand development are summarized.

Development of chimeric receptor activator of nuclear factor-kappa B with glutathione S-transferase in the extracellular domain: Artificial switch in a membrane receptor

Various chimeric receptors have been developed and used for biological experiments. In the present study, we constructed three types of chimeric receptor activator of nuclear factor-kappa B (RANK) with the glutathione S-transferase (GST) protein in the extracellular domain, and stimulated them using newly synthesized chemical trimerizers with three glutathiones. Although this stimulation did not activate these proteins, we unexpectedly found that the chimera named RANK-GST-SC, in which GST replaced a major part of the RANK extracellular domain, activated nuclear factor-kappa B (NF-κB) signaling approximately sixfold more strongly than wild-type RANK without the ligand. The dimerization of extracellular GST is considered to function as a switch outside the cell, and signal transduction then occurs. GST has been widely employed as a tag for protein purification; GST-fusion protein can be conveniently captured by glutathione-conjugated beads and easily purified from impurity. The present study is a pioneering example of the novel utility of GST and provides information for the development of new chemical biology systems.

Evaluating [68Ga]Ga-p14-032 as a Novel PET Tracer for Diagnosis Cerebral Amyloid Angiopathy

Objective: We aimed to investigate the distribution of [⁶⁸Ga]Ga-p14-032, a novel PET ligand that binds to vascular amyloid, in patients diagnosed clinically with probable cerebral amyloid angiopathy (CAA) compared with patients with Alzheimer's disease (AD) and normal controls (NC).

Methods: This longitudinal cohort study was composed of 10 subjects (three probable CAA patients, two AD patients, five NC subjects), recruited from a clinic in China. CAA patients had a history of lobar intracerebral hemorrhage (ICH) and met modified Boston criteria for probable CAA. All participants were aged at least 55 years and underwent [⁶⁸Ga] Ga-p14-032 PET/CT or/and PET/MRI, and the Montreal Cognitive Assessment on initial assessment. Demographics were measured at baseline (diabetes, hypertension, hypercholesterolemia, ischemic stroke, and ICH). Two PET imaging experts reviewed the PET images with cortical standardized uptake value ratio (SUVr) displayed on a color scale and visually classified the images as positive or negative. The mean of SUVr was calculated using the pons as reference.

Results: In CAA patients, PET scans were positive in regions with higher numbers of CMBs. No significant signal was seen in AD subjects or controls. The relative [⁶⁸Ga]Ga-p14-032 retention in the cortex was stronger in patients with CAA than AD and NC (median SUVr 2.68 ± 1.53 vs. 1.77 ± 0.08 and 0.83 ± 0.24).

Conclusions: Our results provide early evidence that the [⁶⁸Ga] Ga-p14-032 PET probe binds preferentially to vascular amyloid and may be a useful tracer for diagnosing CAA.

An Improved Synthesis of the Triethylene Glycol-Substituted 4-(N-Methyl-N-Boc-Amino)Styrylpyridine

An improved five-step synthesis of triethylene glycol-substituted 4-(N-methyl-N-Boc-amino)styrylpyridine (6) is described. Using cost-effective starting materials, the developed synthesis route was synthetic, efficient, and chromatographic purification-free. The key point of the work is the one-pot synthesis of tert-butyl methyl(4-vinylphenyl)carbamate through methylation and elimination in the NaH/THF system. The new synthesis route shows the potential to achieve scaled-up preparation of 6 in the future.

Flexible multidentate benzyldiamine derivatives with high affinity for β-amyloid in cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA) commonly found in the aged is pathologically characterized by β-amyloid (Aβ) deposition in the walls of arteries and capillaries of brain. In this study, four flexible multidentate benzyldiamine derivatives as potential probes for cerebrovascular Aβ deposition were designed and synthesized. In in vitro inhibition assays, the ligands 18-21 displayed high affinities for Aβ aggregates with K_i values of 1.45 ± 0.53 nM, 1.68 ± 0.35 nM, 1.16 ± 0.23 nM and 1.72 ± 0.19 nM, respectively. A significant improvement in the binding affinity over the monomer, compounds 9-12 or benzyldiamine derivatives, demonstrated the applicability of the multidentate approach. The underlying mechanism of these novel Aβ agents was explored by molecular docking technique, which theoretically verified the high affinities of the multidentate benzyldiamine derivatives for Aβ aggregates. Moreover, the molecular masses of the ligands 18-21 are more than 700 Dalton, which are believed to be hardly capable of penetrating blood brain barrier. In this regard, these ligands could be used to distinguish CAA from Alzheimer's disease which is another Aβ-related disorder disease. To convert these ligands to positron emission tomography imaging agents, we attempted to radiosynthesize [¹⁸F]18. Though the radiolabeling was not very successful, the preliminary results suggested that these newly proposed multidentate benzyldiamine derivatives may be used as potential Aβ imaging agents in cerebral amyloid angiopathy.

Syntheses and in vitro biological evaluation of S1PR1 ligands and PET studies of four F-18 labeled radiotracers in the brain of nonhuman primates

A series of seventeen hydroxyl-containing sphingosine 1-phosphate receptor 1 (S1PR1) ligands were designed and synthesized. Their in vitro binding potencies were determined using [32P]S1P competitive binding assays. Compounds 10a, 17a, 17b, and 24 exhibited high S1PR1 binding potencies with IC50 values ranging from 3.9 to 15.4 nM and also displayed high selectivity for S1PR1 over other S1P receptor subtypes (IC50 > 1000 nM for S1PR2-5). The most potent compounds 10a, 17a, 17b, and 24 were subsequently radiolabeled with F-18 in high yields and purities. MicroPET studies in cynomolgus macaque showed that [18F]10a, [18F]17a, and [18F]17b but not [18F]24 crossed the blood brain barrier and had high initial brain uptake. Further validation of [18F]10a, [18F]17a, and [18F]17b in preclinical models of neuroinflammation is warranted to identify a suitable PET radioligand to quantify S1PR1 expression in vivo as a metric of an inflammatory response.

Metal-Free Decarboxylative Alkoxylation of 2-Picolinic Acid and Its Derivatives with Cyclic Ethers: One Step Construction of C-O and C-Cl Bonds

A new strategy for the metal-free decarboxylative alkoxylation of 2-picolinic acid and its derivatives is described. The three-component reaction of 2-picolinic acid or its derivatives, cyclic ethers, and ^tBuOCl proceeded smoothly in the presence of a catalytic amount of p-chloranil to produce 2-alkoxylated pyridines with an ω-chlorine atom in satisfactory to excellent yields. New C-O and C-Cl bonds were generated in one step. The ω-C-Cl bond can be easily transformed to a C-C or C-heteroatom bond, increasing the use of 2-alkoxylated pyridine products in organic synthesis. The electronic property of the substituent linked on the pyridine ring did not influence the reactivity of the 2-picolinic acid substrates.

Al18F-NODA Benzothiazole Derivatives as Imaging Agents for Cerebrovascular Amyloid in Cerebral Amyloid Angiopathy

In this study, we synthesized four novel Al^18/19F-labeled 2-phenylbenzothiazole derivatives conjugated to 1,4,7-triazacyclononane-1,4-diacetic acid via alkyl linkers and evaluated them as imaging agent targets to amyloid-β (Aβ) plaques deposited in the blood vessels of cerebral amyloid angiopathy (CAA) brain. The four ligands exhibited moderate-to-high binding ability to Aβ_1-42 aggregates, of which complex 17 possessing the most potent affinity (K _i = 11.3 nM) was selected for further biological evaluations. In vitro fluorescent staining and in vitro autoradiography studies on brain sections from CAA patients proved that this ligand could label Aβ deposits in blood vessels selectively. In biodistribution study, [¹⁸F]17 can hardly penetrate the blood-brain barrier (brain_2 min = 0.3% ID/g) and displayed a rapid blood washout rate (blood_2 min/blood_60 min = 25.2), which is favorable as CAA imaging agents. In conclusion, this Al¹⁸F-labeled 2-phenylbenzothiazole complex was developed and proved to be a promising CAA positron emission tomography agent.

Rhenium and technetium complexes of thioamide derivatives of pyridylhydrazine that bind to amyloid-β plaques

Age-associated deposition of amyloid-β in cerebral blood vessels, a condition referred to as cerebral amyloid angiopathy, can contribute to stroke and dementia. This research aimed to design new radioactive technetium-99 m complexes that bind to amyloid-β plaques that have the potential to assist in diagnosis of cerebral amyloid angiopathy using single-photon-emitted computed tomography (SPECT) imaging. Six new pyridylthiosemicarbazide ligands containing either benzofuran or styrylpyridyl functional groups that are known to selectively bind to amyloid plaques were prepared. Non-radioactive isotopes of technetium are not available so rhenium was used as a surrogate for exploratory chemistry. The new ligands were used to prepare well-defined [Re-oxo]³⁺ complexes where two pyridylthiosemicarbazide ligands were coordinated to a single metal ion to give bivalent complexes with two amyloid-β targeting functional groups. The interaction of the [Re-oxo]³⁺ complexes with synthetic amyloid-β_1-42 and with amyloid plaques in human brain tissue was investigated. Two ligands were selected to develop methods to prepare their [^99mTc-oxo]³⁺ complexes at the tracer level. These technetium-99 m complexes are likely to be isostructural to their rhenium-oxo analogues.

Amyloid-PET burden and regional distribution in cerebral amyloid angiopathy: a systematic review and meta-analysis of biomarker performance

INTRODUCTION

We performed a meta-analysis to synthesise current evidence on amyloid-positron emission tomography (PET) burden and presumed preferential occipital distribution in sporadic cerebral amyloid angiopathy (CAA).

METHODS

In a PubMed systematic search, we identified case-control studies with extractable data on global and occipital-to-global amyloid-PET uptake in symptomatic patients with CAA (per Boston criteria) versus control groups (healthy participants or patients with non-CAA deep intracerebral haemorrhage) and patients with Alzheimer's disease. To circumvent PET studies' methodological variation, we generated and used 'fold change', that is, ratio of mean amyloid uptake (global and occipital-to-global) of CAA relative to comparison groups. Amyloid-PET uptake biomarker performance was then quantified by random-effects meta-analysis on the ratios of the means. A ratio >1 indicates that amyloid-PET uptake (global or occipital/global) is higher in CAA than comparison groups, and a ratio <1 indicates the reverse.

RESULTS

Seven studies, including 106 patients with CAA (>90% with probable CAA) and 138 controls (96 healthy elderly, 42 deep intracerebral haemorrhage controls) and 72 patients with Alzheimer's disease, were included. Global amyloid-PET ratio between patients with CAA and controls was above 1, with an average effect size of 1.18 (95% CI 1.08 to 1.28; p<0.0001). Occipital-to-global amyloid-PET uptake ratio did not differ between patients with CAA versus patients with deep intracerebral haemorrhage or healthy controls. By contrast, occipital-to-global amyloid-PET uptake ratio was above 1 in patients with CAA versus those with Alzheimer's disease, with an average ratio of 1.10 (95% CI 1.03 to 1.19; p=0.009) and high statistical heterogeneity.

CONCLUSIONS

Our analysis provides exploratory actionable data on the overall effect sizes and strength of amyloid-PET burden and distribution in patients with CAA, useful for future larger studies.

Chalcone Based Homodimeric PET Agent, 11C-(Chal)2DEA-Me, for Beta Amyloid Imaging: Synthesis and Bioevaluation

Homodimeric chalcone based ¹¹C-PET radiotracer, ¹¹C-(Chal)₂DEA-Me, was synthesized, and binding affinity toward beta amyloid (Aβ) was evaluated. The computational studies revealed multiple binding of the tracer at the recognition sites of Aβ fibrils. The bivalent ligand ¹¹C-(Chal)₂DEA-Me displayed higher binding affinity compared to the corresponding monomer, ¹¹C-Chal-Me, and classical Aβ agents. The radiolabeling yield with carbon-11 was 40-55% (decay corrected) with specific activity of 65-90 GBq/μmol. A significant ( p < 0.0001) improvement in the binding affinity of ¹¹C-(Chal)₂DEA-Me with synthetic Aβ42 aggregates over the monomer, ¹¹C-Chal-Me, demonstrates the utility of the bivalent approach. The PET imaging and biodistribution data displayed suitable brain pharmacokinetics of both ligands with higher brain uptake in the case of the bivalent ligand. Metabolite analysis of healthy ddY mouse brain homogenates exhibited high stability of the radiotracers in the brain with >93% intact tracer at 30 min post injection. Both chalcone derivatives were fluorescent in nature and demonstrated significant changes in the emission properties after binding with Aβ42. The preliminary analysis indicates high potential of ¹¹C-(Chal)₂DEA-Me as in vivo Aβ42 imaging tracer and highlights the significance of the bivalent approach to achieve a higher biological response for detection of early stages of amyloidosis.

Recent progress in the development of metal complexes as β-amyloid imaging probes in the brain

In this review, we have focused on the recent progress in metal complexes that are able to bind to β-amyloid (Aβ) species. We have discussed various radioactive complexes of 99mTc, 68Ga, 64Cu, 89Zr, and 111In, which were designed as Aβ imaging agents for positron emission tomography (PET) and single photon emission computed tomography (SPECT) imaging, non-radioactive Re and Ru complexes as Aβ sensors using luminescence methods, and Gd3+ complexes as contrast agents for magnetic resonance imaging (MRI).

Isoflurane anesthesia promotes cognitive impairment by inducing expression of β-amyloid protein-related factors in the hippocampus of aged rats

Isoflurane anesthesia has been shown to be responsible for cognitive impairment in Alzheimer’s disease (AD) and development of AD in the older age groups. However, the pathogenesis of AD-related cognitive impairments induced by isoflurane anesthesia remains elusive. Thus, this study was designed to investigate the mechanism by which isoflurane anesthesia caused AD-related cognitive impairments. Aged Wistar rats were randomly divided into 6 groups (n = 12), 1 control group (CONT) and 5 isoflurane treated (ISO) groups (ISO 0, ISO 0.5D, ISO 1D, ISO 3D and ISO 7D). The CONT group inhaled 30% O₂ for 2 h without any anesthesia. ISO groups were placed under anesthesia with 3% isoflurane and then exposed to 1.5% isoflurane delivered in 30% O₂ for 2 h. Rats in each ISO group were then analyzed immediately (ISO 0) or at various time points (0.5, 1, 3 or 7 day) after this exposure. Cognitive function was assessed using the Morris water maze test. Protein levels of amyloid precursor protein (APP), β-site APP cleavage enzyme-1 (BACE-1) and Aβ₄₂ peptide were analyzed in hippocampal samples by Western blot. β-Amyloid (Abeta) plaques were detected in hippocampal sections by Congo red staining. Compared with controls, all ISO groups showed increased escape latency and impaired spatial memory. Isoflurane increased APP mRNA expression and APP protein depletion, promoting Aβ₄₂ overproduction, oligomerization and accumulation. However, isoflurane did not affect BACE-1 expression. Abeta plaques were observed only in those ISO groups sacrificed at 3 or 7 d. Our data indicate that aged rats exposed to isoflurane had increased APP mRNA expression and APP protein depletion, with Aβ₄₂ peptide overproduction and oligomerization, resulting in formation of Abeta plaques in the hippocampus. Such effects might have contributed to cognitive impairments, including in spatial memory, observed in these rats after isoflurane anesthesia.

Highly specific noninvasive photoacoustic and positron emission tomography of brain plaque with functionalized croconium dye labeled by a radiotracer

Highly-efficient targeting probes are desirable for disease diagnosis and functional imaging. However, most of the current near-infrared (NIR) probes suffer from low signal conversion, insufficient photostability, poor probe specificity, and limited functions. Herein, an NIR ultrahigh absorbing croconium dye for amyloid (CDA) was designed and synthesized to specifically bind to cerebrovascular amyloid without antibody linkage. This unique CDA is able to strongly bind the hydrophobic channels of amyloid beta (Aβ) fiber with a very strong binding energy of -9.3 kcal mol-1. Our experimental results demonstrate that the amphipathic dye with an intense absorption peak at 800 nm generated a significant local temperature surge under low-power laser irradiation. Compared with representative prominent indocyanine green, Prussian blue, and gold nanorods, this probe can produce the strongest photoacoustic signal based on the same mass concentration. Labeled with radioactive 18F, this multifunctional probe allowed for the ultrasensitive photoacoustic tomography (PAT)/positron emission tomography (PET)/fluorescence imaging of Aβ plaques in the brain cortex. Featured with high spatial resolution and optical specificity, PAT was intrinsically suitable for imaging pathological sites on cortical vessels, whereas PET revealed whole-body anatomy with quantitative biodistribution information. Our study shows that a CDA-based functionalized dye aided with PAT and PET is capable of plaque diagnosis and localization.

Novel Bivalent 99mTc-Complex with N-Methyl-Substituted Hydroxamamide as Probe for Imaging of Cerebral Amyloid Angiopathy

Cerebral amyloid angiopathy (CAA) is characterized by the deposition of amyloid aggregates in the walls of the cerebral vasculature. Recently, the development of molecular imaging probes targeting CAA has been attracting much attention. We previously reported the ^99mTc-hydroxamamide (^99mTc-Ham) complex with a bivalent benzothiazole scaffold as a binding moiety for amyloid aggregates ([^99mTc]BT2) and its utility for CAA-specific imaging. However, the simultaneous generation of two radiolabeled complexes derived from the geometric isomers was observed in the ^99mTc-labeling reaction. It was recently reported that the complexation reaction of ⁹⁹Tc with N-methyl-substituted Ham provided a single ⁹⁹Tc-Ham complex consisting of two N-methylated Ham ligands with marked stability. In this article, we designed and synthesized a novel N-methylated bivalent ^99mTc-Ham complex ([^99mTc]MBT2) and evaluated its utility for CAA-specific imaging. N-Methyl substitution of [^99mTc]BT2 prevented the generation of its isomer in the ^99mTc-labeling reaction. Enhanced in vitro stability of [^99mTc]MBT2 as compared with [^99mTc]BT2 was observed. [^99mTc]MBT2 showed very low brain uptake, which is favorable for CAA-specific imaging. An in vitro inhibition assay using β-amyloid aggregates and in vitro autoradiographic examination of brain sections from a Tg2576 mouse and a CAA patient showed a decline in the binding affinity for amyloid aggregates due to N-methylation of the ^99mTc-Ham complex. These results suggest that the scaffold of the ^99mTc-Ham complex may play important roles in the in vitro stability and the binding affinity for amyloid aggregates.

Synthesis and evaluation of copper-64 labeled benzofuran derivatives targeting β-amyloid aggregates

In vivo imaging of β-amyloid (Aβ) aggregates consisting of Aβ(1-40) and Aβ(1-42) peptides by positron emission tomography (PET) contributes to the diagnosis and therapy for Alzheimer's disease (AD). Because (64)Cu (t1/2=12.7h) is a radionuclide for PET with a longer physical half-life than (11)C (t1/2=20min) and (18)F (t1/2=110min), it is an attractive radionuclide for the development of Aβ imaging probes that are suitable for routine use. In the present study, we designed and synthesized two novel (64)Cu labeled benzofuran derivatives and evaluated their utility as PET imaging probes for Aβ aggregates. In an in vitro binding assay, 6 and 8 showed binding affinity for Aβ(1-42) aggregates with a Ki value of 33 and 243nM, respectively. In addition, these probes bound to Aβ plaques deposited in the brain of an AD model mouse in vitro. In a biodistribution experiment using normal mice, these probes showed low brain uptake (0.33% and 0.36% ID/g) at 2min post-injection. Although refinement to enhance brain uptake is needed, [(64)Cu]6 and [(64)Cu]8 demonstrated the feasibility of developing novel PET probes for imaging Aβ aggregates.

Imaging of Cerebral Amyloid Angiopathy with Bivalent (99m)Tc-Hydroxamamide Complexes

Cerebral amyloid angiopathy (CAA), characterized by the deposition of amyloid aggregates in the walls of cerebral vasculature, is a major factor in intracerebral hemorrhage and vascular cognitive impairment and is also associated closely with Alzheimer’s disease (AD). We previously reported ^99mTc-hydroxamamide (^99mTc-Ham) complexes with a bivalent amyloid ligand showing high binding affinity for β-amyloid peptide (Aβ(1–42)) aggregates present frequently in the form in AD. In this article, we applied them to CAA-specific imaging probes, and evaluated their utility for CAA-specific imaging. In vitro inhibition assay using Aβ(1–40) aggregates deposited mainly in CAA and a brain uptake study were performed for ^99mTc-Ham complexes, and all ^99mTc-Ham complexes with an amyloid ligand showed binding affinity for Aβ(1–40) aggregates and very low brain uptake. In vitro autoradiography of human CAA brain sections and ex vivo autoradiography of Tg2576 mice were carried out for bivalent ^99mTc-Ham complexes ([^99mTc]SB2A and [^99mTc]BT2B), and they displayed excellent labeling of Aβ depositions in human CAA brain sections and high affinity and selectivity to CAA in transgenic mice. These results may offer new possibilities for the development of clinically useful CAA-specific imaging probes based on the ^99mTc-Ham complex.

Small Molecule Radiopharmaceuticals - A Review of Current Approaches

Radiopharmaceuticals are an integral component of nuclear medicine and are widely applied in diagnostics and therapy. Though widely applied, the development of an “ideal” radiopharmaceutical can be challenging. Issues such as specificity, selectivity, sensitivity, and feasible chemistry challenge the design and synthesis of radiopharmaceuticals. Over time, strategies to address the issues have evolved by making use of new technological advances in the fields of biology and chemistry. This review presents the application of few advances in design and synthesis of radiopharmaceuticals. The topics covered are bivalent ligand approach and lipidization as part of design modifications for enhanced selectivity and sensitivity and novel synthetic strategies for optimized chemistry and radiolabeling of radiopharmaceuticals.

99mTc(CO)3-Labeled Benzothiazole Derivatives Preferentially Bind Cerebrovascular Amyloid: Potential Use as Imaging Agents for Cerebral Amyloid Angiopathy

Cerebral amyloid angiopathy (CAA) is a disorder affecting the elderly that is characterized by amyloid-β (Aβ) deposition in blood vessel walls of the brain. A series of 99mTc(CO)3-labeled benzothiazole derivatives as potential SPECT imaging probes for cerebrovascular Aβ deposition is reported. Rhenium surrogate displayed high affinities to Aβ aggregates with Ki values ranging from 106 to 42 nM, and they strongly stained Aβ deposits in transgenic mice (Tg) and Alzheimer's disease (AD) patients. In vitro autoradiography on brain sections of Tg and AD patients confirmed that [99mTc]24 possessed sufficient affinity for Aβ plaques, and [99mTc]24 could only label Aβ deposition in blood vessels but not Aβ plaques in the parenchyma of the brain of AD patients. Moreover, [99mTc]24 possessed favorable initial uptake (1.21% ID/g) and fast blood washout (blood2 min/blood60 min=23) in normal mice. These preliminary results suggest that [99mTc]24 may be used as an Aβ imaging probe for the detection of CAA.

Amyloid PET imaging: applications beyond Alzheimer's disease

As a biomarker of beta-amyloid, positron emission tomography (PET) amyloid imaging offers a unique opportunity to detect the presence of this protein in the human body during life. Besides Alzheimer's disease (AD), deposits of beta-amyloid in the brain are also present in other neurodegenerative diseases associated to dementia, such as Parkinson's disease and dementia with Lewy bodies, as well as in other processes affecting brain function, such as cerebral amyloid angiopathy, brain trauma, Down's syndrome and meningiomas, as shown by post-mortem pathology studies. Furthermore, in systemic amyloidosis other organs besides the brain are affected, and amyloid PET imaging may be suitable for the identification of these extra-cerebral amyloid depositions. Finally, the potential use of amyloid PET tracer accumulation in cerebral white matter (WM) as a marker of myelin is being investigated, leading to some promising results in patients with WM lesions and multiple sclerosis. In this article, a review of the ongoing research pointing to a broader application of amyloid PET imaging in clinical practice beyond AD is provided.

Enhancement of binding affinity for amyloid aggregates by multivalent interactions of 99mTc-hydroxamamide complexes

Deposition of amyloid aggregates has been regarded as an early stage of amyloidosis progression. An imaging probe that can image amyloid aggregates enables the early diagnosis of amyloidosis and contributes to the development of new medical therapies. High binding affinity for amyloid aggregates is essential to develop a useful molecular imaging probe. This article describes a new strategy to enhance the binding affinity of imaging agents targeting amyloid aggregates. We designed and synthesized novel (99m)Tc-hydroxamamide ((99m)Tc-Ham) complexes with a bivalent amyloid ligand and evaluated their binding affinity for amyloid aggregates by using β-amyloid peptide (Aβ(1-42)) aggregates as a model. In vitro inhibition assay indicated that bivalent (99m)Tc-Ham complexes had much higher binding affinity for amyloid aggregates than monovalent complexes. In vitro autoradiography using Tg2576 mice showed the specific binding of bivalent (99m)Tc-Ham complexes to Aβ plaques in the mouse brain, as reflected in the results of the inhibition assay. The preliminary results suggest that a new molecular design based on bivalent (99m)Tc-Ham complexes may be reasonable to develop an imaging probe targeting amyloid aggregates.

Transetherification on polyols by intra- and intermolecular nucleophilic substitutions

Transetherification on polyols involving intra- and intermolecular nucleophilic substitutions is reported. Di- or trialkoxide formation of propane-1,3-diol or 2-(hydroxymethyl)propane-1,3-diol derivatives by NaH triggers the reaction via oxetanes formation, where the order to add NaH and a polyol significantly influences the yields of products. It was demonstrated that the protective group on the pentaerythritol skeleton is apparently transferred to the hydrophilic and hydrophobic chain molecules bearing a leaving group in one-step, and a protective group conversion from tosyl to benzyl was successful using a benzyl-appending triol to afford a desired product in 67% yield.

Imaging of cerebrovascular pathology in animal models of Alzheimer's disease

In Alzheimer's disease (AD), vascular pathology may interact with neurodegeneration and thus aggravate cognitive decline. As the relationship between these two processes is poorly understood, research has been increasingly focused on understanding the link between cerebrovascular alterations and AD. This has at last been spurred by the engineering of transgenic animals, which display pathological features of AD and develop cerebral amyloid angiopathy to various degrees. Transgenic models are versatile for investigating the role of amyloid deposition and vascular dysfunction, and for evaluating novel therapeutic concepts. In addition, research has benefited from the development of novel imaging techniques, which are capable of characterizing vascular pathology in vivo. They provide vascular structural read-outs and have the ability to assess the functional consequences of vascular dysfunction as well as to visualize and monitor the molecular processes underlying these pathological alterations. This article focusses on recent in vivo small animal imaging studies addressing vascular aspects related to AD. With the technical advances of imaging modalities such as magnetic resonance, nuclear and microscopic imaging, molecular, functional and structural information related to vascular pathology can now be visualized in vivo in small rodents. Imaging vascular and parenchymal amyloid-β (Aβ) deposition as well as Aβ transport pathways have been shown to be useful to characterize their dynamics and to elucidate their role in the development of cerebral amyloid angiopathy and AD. Structural and functional imaging read-outs have been employed to describe the deleterious affects of Aβ on vessel morphology, hemodynamics and vascular integrity. More recent imaging studies have also addressed how inflammatory processes partake in the pathogenesis of the disease. Moreover, imaging can be pivotal in the search for novel therapies targeting the vasculature.

Cerebral microbleeds: a review of clinical, genetic, and neuroimaging associations

Cerebral microbleeds (microbleeds) are small, punctuate hypointense lesions seen in T2* Gradient-Recall Echo (GRE) and Susceptibility-Weighted (SWI) Magnetic Resonance Imaging (MRI) sequences, corresponding to areas of hemosiderin breakdown products from prior microscopic hemorrhages. They occur in the setting of impaired small vessel integrity, commonly due to either hypertensive vasculopathy or cerebral amyloid angiopathy. Microbleeds are more prevalent in individuals with Alzheimer’s disease (AD) dementia and in those with both ischemic and hemorrhagic stroke. However they are also found in asymptomatic individuals, with increasing prevalence with age, particularly in carriers of the Apolipoprotein (APOE) ε4 allele. Other neuroimaging findings that have been linked with microbleeds include lacunar infarcts and white matter hyperintensities on MRI, and increased cerebral β-amyloid burden using ¹¹C-PiB Positron Emission Tomography. The presence of microbleeds has been suggested to confer increased risk of incident intracerebral hemorrhage – particularly in the setting of anticoagulation – and of complications of immunotherapy for AD. Prospective data regarding the natural history and sequelae of microbleeds are currently limited, however there is a growing evidence base that will serve to inform clinical decision-making in the future.

99mTc-labeled benzothiazole and stilbene derivatives as imaging agents for Aβ plaques in cerebral amyloid angiopathy

^99mTc-labeled probes in this study for the Aβ plaques in the blood vessels of the brain may be used as SPECT imaging agents for the diagnosis of CAA.